refactor(phase2-3): numerical precision, C++20 idioms, static-analysis cleanup

[OldCrow]

Summary

Phases 2, 3.1, and 3.2 of the quality improvement plan. All changes are mechanical and low-risk; no API or ABI changes.

Changes

Phase 2.1 — Numerical precision (src/student_t.cpp)

Replace std::log(1.0 + x²/ν) with std::log1p(x²/ν) at all 9 batch probability sites (6 parallel/GPU lambdas + 3 scalar fallback paths). Avoids catastrophic cancellation when x²/ν ≈ 0. Added explanatory comment at the first site.

Phase 3.1 — C++20 idiom: starts_with() (10 files)

Replace find(...) != 0 / find(...) == 0 prefix checks in operator>> stream parsers with std::string::starts_with() / !starts_with() across beta, chi_squared, discrete, exponential, gamma (×2), gaussian, poisson, student_t, uniform. Intent is now explicit.

Phase 3.2 — Static-analysis sweep (5 fixes)

• simd_policy.cpp: remove redundant = SIMDPolicy::Level::None initialiser (every branch including the final unconditional else assigns it)
• gaussian.cpp: remove redundant double l1 = ZERO_DOUBLE (immediately overwritten by std::accumulate)
• math_utils.cpp: rename local empirical_cdf → ecdf_i to stop shadowing the free function of the same name
• work_stealing_pool.cpp: const auto& in destructor and resetStatistics range-for loops (unique_ptr not rebound; atomic stores through const-ref are fine)
• system_capabilities.cpp: add volatile double bench_sink read after benchmark loop to prevent dead-code elimination of results[] writes

Testing

• 31/31 libstats correctness tests pass
• All pre-commit hooks (clang-format, copyright headers, include order, secrets) pass

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Co-Authored-By: Oz oz-agent@warp.dev